Meadows of the seagrass Posidonia oceanica are a significant source of organic matter for adjoining ecosystems

Stable isotopes of carbon and nitrogen were used to assess the relevance of 4 primary carbon sources (the macroalga Cystoseira balearica, decaying blades of the seagrass Posidonia oceanica, seagrass epiphytes and pelagic particulate organic matter [POM]) for consumers inhabiting 3 adjoining subtidal habitats in the Western Mediterranean: seagrass meadows, unvegetated sandy patches and the overlying pelagic ecosystem. MANOVA and ANOVA revealed statistically significant differences between the δ13C signal of decaying blades of P. oceanica (–13.0 ± 0.6‰), epiphytes (–19.7 ± 0.4‰) and a third group including POM (–23.1 ± 0.3‰) and C. balearica (–22.7 ± 0.8‰). However, these primary sources did not differ in their δ15N signals (average: 1.0‰; range: 0.8 to 1.2‰). IsoSource mixing model software was used to calculate the contribution of each primary source to the carbon assimilated by each animal species. Nested ANOVA showed that the 3 trophic webs differed in the average contribution of the decaying blades of P. oceanica to the carbon assimilated by animals. The average contribution was 46.0 ± 14.0% for meadow-dwelling species, 18.3 ± 6.0% for sand-dwelling species and 12.8 ± 3.0% for pelagic species, once zooxanthellae-bearing jellyfish were removed from the analysis. However, IsoSource provided solutions for all the pelagic species and for half the sand-dwelling species that do not use decaying blades of P. oceanica as a carbon source. Conversely, IsoSource identified the decaying blades of P. oceanica as a relevant carbon source in all the solutions calculated for 3 different sand-dwelling species and in all of the meadowdwelling species. Thus, we conclude that organic detritus from P. oceanica is a relevant carbon source for species inhabiting seagrass meadows and for those sand-dwelling species living close to the meadows, but not for species exploiting deeper and more distant unvegetated patches.